--- Comment #1 from Jeremy Harris <jgh146exb [at] wizmail> 2012-08-19 16:02:04 --- If control=no_mbox_unspool is not sufficient, raw access to the spool files is possible for custom actions - use $spool_directory and $message_exim_id with reference to http://exim.org/exim-html-current/doc/html/spec_html/ch55.html

It's not clear that there are enough use cases to justify the addition of the feature. Another approach for the example case would be a fake-reject with delivery to the alternate file.